1. Field of the Invention
The present invention relates to machines for the injection-molding of plastic or metallic parts and articles, and more particularly to the die closing units of such injection molding machines by means of which injection molding dies are opened and closed and maintained in the closed position under elevated closing pressure, through the action of one or several hydraulic cylinders.
2. Description of the Prior Art
An injection molding die can either be pushed closed or pulled closed. Accordingly, one distinguishes among the die closing units of injection molding machines between push-type die closing units and pull-type die closing units. In both cases, the injection molding die, which normally consists of two die halves, is mounted on and between two die carrier plates or blocks of which one is normally stationary and the other is longitudinally movable.
In the push-type die closing unit, the hydraulic cylinder, or cylinders, are arranged longitudinally behind the movable die carrier plate, pushing the later against the stationary die carrier plate, thereby closing the molding die between them. In order to obtain the necessary rigidity in such a structural configuration, and in order to minimize the strain on the supporting machine frame, this type of die closing unit has at least two tie rods extending between the stationary die carrier and the stationary part of the hydraulic cylinder unit, thereby eliminating any bending moment on the supporting machine frame. These tie rods, either two or four of them, are parallel longitudinally extending rods surrounding the injection molding die.
In the case of a pull-type closing unit, the hydraulic cylinder, or cylinders, are arranged longitudinally behind the stationary die carrier plate, reaching beyond the injection molding die to the movable die carrier plate, and pulling the latter towards the stationary die carrier plate in the die closing sense. Normally, it is the piston rods of the hydraulic cylinders which are extended longitudinally to reach all the way to the movable die carrier plate. Accordingly, such a die closing unit has almost always two or four hydraulic cylinders whose elongated piston rods serve as tie rods in the sense of transmitting pulling forces between the die carrying members of the die closing unit.
It can therefore be seen that the two basic types of die closing units have in common that their longitudinally oppositely positioned die carrier plates are linked together by means of rod members whose longitudinal tension equals the closing pressure which is to be generated on the injection molding die, especially during the injection operation, when plastified raw material is injected into the closed injection molding die at high pressure. In the context of this disclosure, therefore, these rod members will be generally referred to as tie rods, whether they are in fact longitudinally moving piston rods or stationary rods extending between two stationary members of the die closing unit.
An injection molding die is commonly composed of at least two heavy plates which are mounted axially between the stationary and movable die carrier plates, in the space between the tie rods; at least two such tie rods are always present. Between the several separable die plates are arranged suitable guide means and/or centering means, mostly axially protruding pilot members in one plate engaging matching recesses in the adjoining plate. Since it is a general requirement that the injection molding dies are removable from their respective die carrier plates, either for servicing or for replacement with a different die assembly, the outside dimensions of these die assemblies are determined by the access space which is available between the longitudinal tie rods. It follows that the transverse distance between the tie rods constitutes a dimensional limitation on the dimensions of the injection molding die, and hence on the effectively available cross-sectional area of the injection molding cavity. This limitation applies to at least one dimensional axis in the case of only two tie rods, but it limits both axes in the case of four tie rods. The maximum acceptable dimensions of the injection molding die are thus considerably reduced from the dimensions of the die carrier plates. The reverse distances between the tie rods, on the other hand, cannot be increased at will, because this would require larger and much heavier die carrier plates and, in the case of the pull-type die closing unit, a larger heavier cylinder mount, because the several hydraulic cylinders are normally arranged inside an integrally cast block.
The prior art in this field, therefore, was limited to the use of injection molding dies whose dimension along at least one axis was determined by the transverse opening between two tie rods (see German Pat. No. 1,300,201 ). An improved utilization of space is suggested in the German Pat. No. 1,265,390, where the space between the tie rods is utilized more efficiently, by combining the guide elements with the centering elements in a coaxial arrangement. A common shortcoming of these past attempts at a better utilization of the die mounting space in die closing units is that they fail to utilize that area which lies outside the area defined by an extension of the maximum opening between the tie rods, because the die plates have to be inserted through that opening.